In recent years, there has been widely used a storage system using a plurality of storage devices, such as HDDs (Hard Disk Drives). Many of such storage systems have redundant controllers for controlling access to the storage devices in order to improve reliability of data access to the storage devices. For example, there has been proposed a storage system configured such that if both of duplexed controllers are stopped due to a hardware error, the hardware is recovered from the error having temporarily occurred by executing processing for powering off and on the both systems.
Further, in the storage systems, in general, record control in which data is redundantly stored in two or more storage devices is performed using a RAID (Redundant Arrays of Independent Disks) technique, whereby the safety of recorded data is improved.
Furthermore, in the storage systems which redundantly control data, if one of storage devices suffers from a failure, data stored in the faulty storage device is rebuilt, and is stored in another storage device. This process is generally referred to as “the rebuild process”. By executing the rebuild process, redundancy of the data is recovered.
Many of storage systems have a spare storage device provided therein, referred to as the “hot spare”, and often execute the rebuild process using the hot spare. On the other hand, there are also other storage systems that do not use a hot spare, but execute the rebuild process for a new storage device when a faulty storage device is replaced by the new storage device. For example, there has been proposed a storage system configured such that when an error has occurred in one of storage devices managed by RAID 5, parity is rebuilt for a storage device having replaced the faulty storage device, by copying data not from a storage device within the same parity group, but from a storage device at the same location in the mirror configuration for snap shot.
See, for example, Japanese Laid-Open Patent Publication No. 2004-206239 and Japanese Laid-Open Patent Publication No. 2002-108571.
By the way, in a storage system, if one of storage devices suffers from a failure in a state in which data has been duplexed, such as a state managed by RAID 5, the rebuild process is executed in a state in which the redundancy of the data is lost. During the rebuild process in the state in which the redundancy of the data is lost as mentioned above, there can further occur a failure in reading out data from still another storage device. This readout failure may be caused e.g. by a partial defect on the surface of a disk medium. If the readout of data from the still other storage device fails in the state in which the redundancy of the data is lost, the data is lost.